1. Field of the Invention
The present invention relates to a motor driver for driving a spindle motor for use in, for example, an FD (floppy disk) drive apparatus, and particularly to a motor driver having a braking system for applying braking to the rotation of a motor.
2. Description of the Prior Art
Conventionally, a motor driver for driving a spindle motor for use in a CD-ROM drive apparatus or for driving a capstan for use in a magnetic tape drive mechanism has a braking system for applying braking to the rotation of the motor. Now, how braking is applied to the rotation of a motor will be described with reference to FIG. 2.
First, both of the upper and lower power transistors TU and TL Sconstituting each of three pairs of power transistors connected in series between the supply voltage V.sub.CC and the reference voltage GND are turned off. As a result. the supply of electric currents to the three coils U, V, and W provided one for each phase in the motor is shut off. Then, in each pair, the upper power transistor TU (i.e. the one arranged on the higher-potential side) is turned off and the lower power transistor TL (i.e. the one arranged on the lower-potential side) is turned on, or alternatively the upper power transistor TU is turned on and the lower power transistor TL is turned off. As a result, braking is applied to the rotation of the motor.
However, in a conventional braking system, braking starts being applied immediately after the supply of electric currents has been shut off. In this system, if the electric currents are shut off when the upper power transistor TU is on and the lower power transistor TL is off, there is a possibility that the upper and lower power transistors TU and TL are turned on simultaneously. This may lead to destruction of a circuit element or to malfunctioning of a related circuit.
Moreover, no conventional motor driver is designed to release braking automatically even when the rotation rate of the motor drops down to a predetermined rate after braking has started being applied to the rotation of the motor. This makes it necessary to control the operation of the motor driver by the use of a control circuit that can monitor the rotation rate of the motor so that, when the rotation rate drops down to a predetermined rate, the motor driver will be fed with an instruction that requests releasing of braking. This makes the burden on the control circuit heavier.
Moreover, a conventional motor driver is designed to activate a braking system simply in response to a control signal that requests its activation, and thus is not equipped to prevent malfunctioning of the braking system. This makes it necessary to design the control circuit to manage strictly the control signal fed to the motor driver in order to prevent malfunctioning of the braking system that belongs to the motor driver. This makes the burden on the control circuit even heavier.
Moreover, in a conventional FD drive apparatus, a spindle motor is driven by the use of a motor driver that does not have a braking system. This is because, in a conventional FD drive apparatus, where the spindle motor is rotated at 360 rpm for high-rate rotation and at 300 rpm for low-rate rotation, the difference between the rates for high and low rotation is small, and therefore deceleration from the high rate to the low rate can be achieved simply by inertial rotation, i.e. without using any braking system. However, in a recently developed high-capacity FD drive apparatus, where the spindle motor is rotated at 3600 rpm for high-rate rotation and at 300 rpm for low-rate rotation, deceleration from the high rate to the low rate takes too much time if attempted simply by inertial rotation, and therefore a braking system is indispensable.